i want to deliver a top quality digital signal for uncompressed music, probably from the usb port of an applemac to my primare sound processor/pre-amp. this has optical and electrical digital inputs; also RS-232. i want to use the dac within the sound processor as this has several useful surround sound modes as well as 2 channel stereo. i plan to buy a second-hand applemac (i have my eye on a powerbook) to dedicate to the task. can anyone suggest what would be the best way to link the computer and the sound processor?

Apart from what Sammie suggests, if you want to spend some money, I would first try Rhanson's suggestion. The HagUSB is excellent and yes you can choose between an XLR or RCA out and it costs about a 100 bucks